Motor Reflexes (Karius)

Question 1

What is the general purpose of a reflex?

Answer 1

Protective, required movement (can happen with no voluntary control of muscle)

Question 2

Features of a reflex

Answer 2

Very specific motion and very fast

Also very precise and can look like voluntary movement. It is much faster than voluntary movement

Question 3

Reflexes vs Voluntary

Answer 3

Reflexes do not required cortex involvement
Involuntary
Initiated by sensory input with no higher cognition
Fixed circuitry
Fast and high specificity

Question 4

Myotatic reflex

Answer 4

Contraction of stretched muscle to protect from tearing due to stretch (basically opposite of golgi tendon)

Question 5

What initiates the myotatic reflex?

Answer 5

Muscle spindle.

Reflex is monosynaptic and goes out to the same segment of the spinal cord

Question 6

Draw the skeletal muscle including fusiform capsule, muscle spindle, intra and extrafusal fibers. Label the sensory and motor components and where the nuclear bag and chains are located.

Answer 6

Ok

Question 7

Characteristics of the sensory component of the muscle spindle

Answer 7

Not contractile, sensitive to length

Contains nuclear bag and nuclear chain

Question 8

Primary afferent vs secondary afferent

Answer 8

1a/1b fibers (large myelinated) vs group II fiber (small, myelinated)

• innervates both the bag and the chain
• innervates the chain

Question 9

1a fiber makes the sensory component sensitive to…

Answer 9

Length of muscle, and how fast the length is changing

Question 10

What happens to the sensory component when there is stretch (change in length) detected?
How about if the stretch is fast?

Answer 10

Frequency of AP is increased compared to unstretched.

Since 1a fibers are sensitive to speed, the AP frequency will increase more

Question 11

What does Group II fiber make sensory component sensitive to?

Answer 11

Only length of muscle, not speed

Question 12

What innervates the motor component?

What does it control?

Answer 12

Gamma motor neuron,

controls the intrafusal fibers, thereby controling the length of the sensory portion

Question 13

What happens when the motor component stretches the sensory portion?

Answer 13

Sensory part becomes more sensitive to more stretch due to increase sensitivity of the 1a and II fibers to stretch (stronger stretch response)

Question 14

Describe the myotatic reflex pathway

Answer 14

1a afferent enters the dorsal root > release EAA at synapse > synapse to alpha motor neuron > contract the stretched target muscle

1 synapse system

Question 15

Explain what happens to the antagonist muscle during the myotatic reflex:

Answer 15

1a afferent synapses to an inhibitory neuron by releasing EAA > inhibitory interneuron will release inhibitory GABA > synapse to the alpha motor neuron of the antagonist muscle > inhibits it and it generates fewer action potentials > relaxing (stretching) the muscle

*1a afferent also sends info to the cortex informing the brain of the reflex event

Question 16

What is the role of antagonist stretching in the myotatic reflex?

Answer 16

If antagonist is relaxed, it cannot oppose the contraction of the agonist, so the myotatic reflex can happen

Question 17

How is the Golgi tendon reflex different from the myotatic reflex?

Answer 17

Relaxes a contracted muscle suddenly. Purpose is to protect the muscle from damage if it contracts too strongly

Question 18

Characteristics of the Golgi tendon reflex?

Answer 18

Initiated by the golgi tendon organ

2 synapses

Question 19

What are golgi tendon organs?

Answer 19

1b fibers that innervate the tendons

Requires contraction of the target muscle (lots of action potentials that increase with tension).

Question 20

Explain the pathway of the golgi tendon reflex

Answer 20

1b afferent enters the spinal cord via the dorsal horn > release of EAA on inhibitory interneuron > GABA is released by inhibitory interneuron and synapses on alpha motor neuron of the contracting muscle > inhibits it and causes the muscle to relax suddenly

*GTR is a high threshold reflex. Needs lots of contraction to stimulate the interneuron

Question 21

Cortical reflexes:

Answer 21

hopping reaction

placing reaction

Question 22

Brainstem/midbrain reflexes

Answer 22

vestibular 
righting 
suckle
yawn 
eye and head movements

Question 23

spinal reflexes

Answer 23

stretch
golgi tendon
crossed extensor

Question 24

What happens to alpha motor neurons after a spinal cord transection?

Answer 24

Spinal shock. Loss of input from the upstream neurons/cortex. Loss of all reflexes below the lesion

Question 25

What is axonal sprouting and how does it occur?

Answer 25

5HTC receptor is expressed (self-activating)

Question 26

What is the reason for hyperactive reflexes after brain injury?

Answer 26

Brain inhibitory region needs input from the cortex to work. Without this, it is unable to inhibit gamma motor neurons. BFC activates, but no inactivation, gamma motor neurons firing rate is altered = more active reflex

Question 27

Patient presents with flexed upper limbs, extended and internally rotated lower limbs. Position seems to be dependent on head position.

What condition does this refer to?
Where is the lesion?

Answer 27

Decorticate posturing

Lesion is above the red nucleus

Question 28

Patient presents with extended upper and lower limbs. Position seems to be independent of head position.

What condition does this refer to?
Where is the lesion?

Answer 28

Decerebrate posturing

Lesion is caudal to the red nucleus. Loss of input from areas rostral to pons.

Question 29

Patients with spasticity present with what symptoms?

What is the usual cause?

Answer 29

resisting passive muscle stretch.
Hyperactive myotatic reflex (due to overactive gamma motor neurons).

Cortex damage > inhibition of brainstem inhibitory region

Question 30

Patients with rigidity present with what symptoms?

What is the usual cause?

Answer 30

Muscle contraction in the absence of stimuli due to alpha motor neurons continuously firing

Cortex damage > inhibition of medullary influence to alpha motor neurons > overactive alpha motor neurons